Description of the Conventional Technique
Conventionally, a microwave oven, which is one example of a heat cooker, uses a humidity sensor with a semiconductor to detect cooking conditions and to control the heat source. In this example, the steam from the food which is heated dielectrically bonds to the semiconductor element in the humidity sensor, and the source of the electromagnetic waves is either controlled or shut off by the electric signal generated by the element. However, this system had a reliability problem when cooking food due to oil and meat juice particles, not just the steam from food, which polluted the semiconductor. Even when periodically a burning process of this pollutant was attempted using a heater or the like, the electric signal from the sensor gradually changed, possibly becoming the cause for its deterioration or malfunction.
In contrast to a humidity sensor with a semiconductor to detect humidity in the air, a method of measuring humidity by sonic waves is known, which was reported in the Kohkai Sho 48-34585. This method utilizes the characteristics that if there is a constant temperature, the velocity of the sonic waves going through the atmosphere becomes higher as the humidity becomes higher. This method of measuring humidity solves the problem of sensor pollution. However, there is a problem of measuring the humidity under the condition when the temperature change is substantial, such as in a cooker, because the velocity change of the sonic waves is more affected by the temperature change than the humidity change.